Carburetor adjusting method

ABSTRACT

A method for optimizing the performance of an internal combustion engine carburetor to minimize the amounts of carbon monoxide and hydrocarbons in the engine exhaust gases while still providing for an efficiently operating engine is disclosed. The method involves adjusting the air-fuel mixture to the carburetor so as to minimize the amount of hydrocarbons in the engine exhaust gases.

United States Patent Voelz 5] Feb.8,1972

[54] CARBURETOR ADJUSTING METHOD [72] Inventor: Frederick L. Voelz,Orland Park, Ill.

[73] Assignee: Atlantic Richfield Company, New York,

[22] Filed: Nov. 4, 1970 [21] Appl. No.: 86,930

[52 U.S.Cl. ..123/119n 511 lnt.Cl ...F02m3/00 [5s fieldofSeai-ch..123/119 OTHER PUBLICATIONS Internal Combustion Engines by Gill, Smith,& Ziurys Lincoln 1968 Shop Manual Supp. Lincoln Mercury Div. Ford MotorCo. (pp. 10-9, 1040, 10-1 I) The Automobile & Air Pollution Part ll U.S.Dept. of Comm. 12-67 (pp. 36, 37. 38 & 39)

Primary Examiner-Laurence M. Goodridge Attorney-Robert J. Mawhinney andThomas J. Clough [57] ABSTRACT 10 Claims, No Drawings CARBURETORADJUSTING METHOD This invention relates to a method for reducing airpollution from intemal'combustion engines. More particularly, thisinvention relates to a method for optimizing the operation of thecarburetor of an internal combustion engine'so as to achieve both asmooth, efficient running engine and a minimum amount of harmful engineexhaust emissions.

The internal combustion engine manufactures useful power by theexplosive combustion of fuel, normally of the hydrocarbon type, such asnatural gas, gasoline, kerosene, diesel fuel, etc., and oxygen, normallytaken from the air; Prior to combustion, the fuel and oxygen areintimately mixed in controlled amounts. This mixing occurs in acarburetor. The carburetor normally is equipped with at least one idlemixture screw the position of which can be adjusted (e.g., by turningthe screw in or out) to vary the air-fuel ratio (by varying the amountof either fuel or air) to the engine combustion chambers. In animportant sense, the operational efficiency of the internal combustionengine depends on the carburetor being properly adjusted. If the enginereceives an air-fuel mixture overly lean in fuel, the engine will runrough due to not being able to sustain combustion in one or more of thecylinders, i.e.', one or more of the cylinders will miss. On the otherhand, if the engine receives a mixture overly rich in fuel, the enginewill be wasteful, uneconomical and tend to develop excessive harmfuldeposits.

Many methods for adjusting carburetors have sought to control theair-fuel ratio at the value specified by the engine manufacturer. Thesemethods, and in fact the manufacturerset air-fuel ratios, have, as aprimary goal, the smooth operation of the engine. None of these methodsseek to adjust the carburetor so as to give a smooth, efficient runningengine which emits a minimum amount of air pollutants, such as carbonmonoxide and hydrocarbons. in fact, engines may emit larger amounts ofthese pollutants after having the air-fuel ratio adjusted to themanufacturer-set value than before the adjustment. This isdisadvantageous since internal combustion engine exhaust emissions, suchas from automobiles, trucks and other transportation means, contributesignificantly to the airpollution problem.

Therefore, one of the objects of the present invention is to provide animproved method for optimim'ng the operation of an internal combustionengine carburetor. Other objects and advantages of the present inventionwill become apparent hereinafter. v

The present invention comprises a method for optimizing the operation atidle of an internal combustion engine carburetor, equipped with at leastone idle mixture screw the position of which may be adjusted (e.g.,turned) to vary the airfuel ratio to the engine combustion chambers. Inone embodiment, the present invention is a method which comprises:

i. analyzing for the hydrocarbon concentration of the exhaust gases fromthe engineequipped with the carburetor 7. setting the position of theidle mixture screws so that the corresponding air-fuel ratio provides anengine exhaust gas hydrocarbon concentration within a range from zero toabout 50 p.p.m. above the minimum hydrocarbon conoentration observed insteps l), 3) and (6).

Note that in step (7) it may be unnecessary to readjust the position ofthe idle mixture screws if the last idle mixture screw adjustment instep (6) results in an increase in hydrocarbon concentration of about 50p.p.m. or less. In this situation the last idle mixture screw positionset in step (6) gives aproper carburetor adjustment according to themethod 7 of the present invention and this modification of the method isexpressly included in the scope of the present invention.

In order to insure a smooth, efficiently running engine whilemaintaining substantial pollution control benefitsfrom the presentinvention, it is preferred to adjust the carburetor to feed a slightlyrich (rather than a slightly lean) air-fuel mixture to the engine.Therefore, in a preferred embodiment of the present invention, step (7)of the above method involves adjusting the position of the idle mixturescrews so that the corresponding air-fuel ratio contains a maximumproportion of fuel'and provides an exhaust gas hydrocarbon concentrationa range from zero to about p.p.m. above the miniminh hydrocarbonconcentration observed in steps (l), (3) and (6). In a more preferredembodiment of the present invention, step (7) involves adjusting theposition of the idle mixture screws so that the corresponding airfuelratio provides the minimum exhaust gas hydrocarbon concentrationobserved in' steps (I), (3) and (6). Due to hydrocarbon analysislirnitations and/or the characteristics of the particular carburetorbeing optimized, more than one air-fuel ratio may provide at engineexhaust gas having the same minimum hydrocarbon concentration observedin steps (1 (3) and (6). Therefore, in the most preferred embodiment ofthe present invention, step (7) comprises adjusting the idle mixturescrews so that the corresponding air-fuel ratio contains a minimumproportion of fuel and provides the minimum exhaust gas hydrocarbonconcentration observed in steps l (3) and (6 In practicing the method ofthe present invention, it is possible I that the first two (or more)exhaust gas analyses will show the being optimized, the exhaust gasesbeing sampled while the engine is operated at essentially constantconditions on the idle carburetion circuit at normal engine operatingtemperatures;

2. adjusting the position of the idle mixture screws by a smallincrement to change the air-fuel ratio;

3. repeating step l with the air-fuel ratio being changed as in step(2).

4. adjusting the position of the idle mixture screws by a smallincrement to change the air-fuel ratio in the direction of decreasinghydrocarbon concentration, said direction of decreasing hydrocarbonconcentration being determined by comparing the hydrocarbonconcentrations obtained in steps l and (3 repeating step (1 with theair-fuel ratio being changed as in step (4);

tained in the immediately preceding analysis; and

same hydrocarbon concentration. in this instance, it is necessary tofurther adjust the idle rnixture screws by a small more ment and againanalyze the exhaust gases in order to determine the direction ofdecreasing hydrocarbon concentration. This modification is expresslyincluded in the scope of the present invention as outlined in step 1 to(7) above.

In' order that the minimum exhaust gas hydrocarbon concentrationobtainable be approximated by practicing the method of the presentinvention, it is necessary that the position of the idle mixture screwsbe adjusted in small increments. In most cases, this means that the idlemixture screws are adjusted, i.e., turned, less than about one-half turn(e.g., one-fourth turn); preferably less than about one-fourth turn(e.g., one-eighth turn); and more preferably less than about one-eighthturn (e.g., one-sixteenth turn) between exhaust gas hydrocarbonanalyses. Of course, while practicing the method repeating steps (4) and(5) at least until an analysis shows of the present invention it ispossible, and may even be preferred, to adjust the position of the idlemixture screws by different sized small increments. For example, as youap' preach the minimum exhaust gas hydrocarbon concentration, the idlemixture screw adjustment between analyses may be made smaller in orderto fine tune the carburetor.

It is preferred to practice the present invention with a carbure'torwhich initially provides an overly rich air-fuel mixture. One reason forthis is because an engine operating on an overly rich air-fuel mixturewill run smoothly and, therefore, provide an exhaust gas having aconsistent hydrocarbon concentration, whereas an engine operating on alean air-fuel mixture tends to have uneven fuel combustion and providesan exha'iist gas having an inconsistent (i.e., nonreproduciblehydrocarbon concentration. Also, since the exhaust gas hydrocai'bonconcentration is generally less sensitive to changes in idle mixturescrew position with an overly rich airfuel mixture, approaching theminimum hydrocarbon concentration from the rich side is more convenientthan from the lean" side. If the carburetor is known to initiallyprovide an overly rich air-fuel mixture, it is preferred that step (2)of the method as outlined above comprise adjusting the position of theidle mixture screws by a small increment to increase the air-fuel ratio(i.e., reduce the fuel proportion or increase the air proportion of theair-fuel mixture).

Engine operation with an overly rich air-fuel mixture can be assured byincreasing the fuel proportion beyond that needed to give a smoothrunning engine. This may be done in many ways. For example, one mayadjust the air-fuel ratio to the manufacturer-set value and thenincrease the fuel proportion (or reduce the air proportion). Altemately,one may lightly seat the idle mixture screws (i.e., seal off completelythe fuel or air to the engine without applying excessive force to theidle mixture screws which might cause permanent damage to thecarburetor) and then turn open the idle mixture screws to a point which,from experience, is known to give an overly rich air-fuel mixture. Thislatter method is preferred since lightly seating the idle mixture screwstends to cause certain carburetor deposits to be jarred loose, thusproviding a cleaner carburetor which when adjusted will maintain thisproper adjustment for a substantial period of time.

While practicing the method of this invention, the engine choke system,if any, should be completely open so that this system does not restrictthe flow of air to the engine. A completely open choke system can beobtained by operating the engine for a sufficient length of time toachieve normal operating temperature. Of course, the choke system canalso be manually disengaged from the engine so that it will notinterfere with the testing procedure. However, the engine should be runat normal operating temperatures when the carburetor is being adjusted.Generally, normal operating temperatures for internal combustion enginesare from about 170 to about 240 F. (engine block temperature).

In order to achieve maximum benefit from the invention, i.e., a propercarburetor adjustment for a reasonably long period of time, it may benecessary to perform certain maintenance on the engine prior toadjusting the carburetor. In particular, it is preferred to adjust acarburetor that is free of deposits and/or other contaminants that mightcause fuel or air blockages and thus disrupt the carburetor adjustment.Various components of the air and fuel intake systems, e.g., carburetorair filter element, positive crankcase ventilation system (if any),etc., as well as the carburetor itself can be inspected and maintenanceperformed if necessary prior to making the carburetor adjustment.

Any carburetor which is used to provide a mixture of fuel and combustionair to an internal combustion engine is capable of being adjustedaccording to the method of the present invention. These carburetors mayhave one or a plurality (e.g., two, four) of mixing chambers or jets.

In order to practice this invention, the internal combustion engineequipped with the carburetor to be adjusted is operated on the idlecarburetion circuit. Idle carburetion circuit operation is normallyachieved at engine speeds from about 400 rpm. to about 1,100 rpm. Theengine operated in the practice of this invention may be the enginenormally equipped with the carburetor to be adjusted, or it may be anyother internal combustion engine capable of employing this carburetor toprovide a controlled fuel-air mixture. For convenience, speed andaccuracy of adjustment, it is preferred that the test engine be theengine normally equipped with the carburetor to be adjusted. The engineidle speed is preferably set according to the manufacturer'sspecification both before and after the carburetor adjustment. Includedamong the internal combustion engines used in the practice of thisinvention are those engines operated in association with transportationmeans such as automobiles, trucks, etc. Internal combustion engines notassociated with transportation means can, of course, also be used in thepractice of this invention. Engines which are used and/or designed fortesting purposes are also suitable. In fact, any internal combustionengine, including two-cycle engines, four-cycle engines, rotary pistondriven engines and turbine engines, which uses a carburetor to provide aair-fuel mixture can be utilized in the practice of the presentinvention.

The hydrocarbon concentration of the exhaust gases may be analyzed forin any conventional manner known to the art. Ineluded among theseconventional analytical methods are: gas chromatography, massspectrometry, and infrared spectrometry. Because of the speed andaccuracy of the final analysis, it is preferred to utilize infraredspectrometry in the practice of the present invention. In particular,the use of nondispersive infrared (NDIR) analyzers is preferred in thepractice of this invention. These infrared analyzers operate on theknown principle that hydrocarbons absorb infrared energy having aspecific wavelength. When infrared energy is sent through a stream ofengine exhaust gas, a certain amount of energy is absorbed by thehydrocarbons in the gas stream. The amount of absorbed energy has adirect relationship to the volume concentration of hydrocarbon in theexhaust gas. By comparing, normally using electronic means, the amountof infrared energy of the wavelength absorbed by hydrocarbons remainingwith the original amount of infrared energy of this wavelength, it ispossible to determine the amount of hydrocarbon in the exhaust gas. Thistype of infrared analyzer can be packaged as a relatively portableinstrument. This analyzer mobility is an additional reason for prefem'nginfrared spectrometry for analyzing the engine exhaust gases forhydrocarbon concentration.

When adjusting carburetors of internal combustion engines that areassociated with automobiles and other motor vehicles, it is preferred tosample the engine exhaust gases for hydrocarbon analysis from theexhaust system effluent, i.e., tailpipe effluent. Since the engineexhaust system, (i.e., mufiler, tailpipe, etc.) is subject to greatwear, the possibility of gas leaks exists. Therefore, in order to insurethe accuracy of the tailpipe effluent hydrocarbon analysis, it ispreferred that if the tailpipe effluent is used as a source of exhaustgas samples, the engine exhaust system be tested for gas leaks at somepoint during the practice of this invention. It is preferred that theleak testing take place prior to any carburetor adjusting taking place.

The exhaust system leak testing can be accomplished in any conventionalmanner, for example, visual inspection of the exhaust system. However,the preferred method of leak testing is to analyze the tailpipe effluentfor oxygen concentration. It is well known that the exhaust gases from aconventional fourcycle internal combustion engine (standard automobileengine) contain about 1 to about 4 percent by volume of oxygen. Anysignificant deviation, for example, by at least about 3 percent byvolume from the upper limit of this range, indicates a leak in theengine exhaust system. Exhaust gases from engines which are equippedwith air injection emission control devices normally contain betweenabout 7 to about 20 percent by volume of oxygen and, therefore, may bedeemed insensitive to the oxygen analysis" method of testing for airleaks. The oxygen concentration can be determined by any conventionalmethod, such as amperometric methods, magnetic susceptibility methods,mass spectrometry and gas chromatography. The preferred methods ofoxygen analysis are the amperometric methods.

The following procedure may be used in the practice of the presentinvention to optimize the performance of a single barrel carburetor.This type of carburetor has only one idle mixture screw which can beturned, for example, counterclockwise to allow more fuel to flow to theengine and, conversely, clockwise to reduce the amount of fuel to theengine. While practicing the present invention, the engine is operatedat essentially constant conditions on the idle carburetion circuit atnormal operating temperatures. The idle mixture screw may be lightlyseated and then adjusted so as to provide an overly rich air-fuelmixture to the engine. At this point, the engine exhaust gases areanalyzed for hydrocarbon content and the concentration is noted andrecorded. Next, the idle mixture screw is turned clockwise by a smallincrement, for example, onewfourth turn, to reduce the amount of fuel tothe engine and the exhaust gases are again analyzed for hydrocarbonconcentration. The steps of clockwise adjustment of the idle mixturescrew and analysis of the engine exhaust gases are re- 5 ficient runningand the amounts of carbon monoxide and hydrocarbon emitted in the engineexhaust gases are minimized.

The same general procedure can be followed in adjusting carburetorshaving a plurality of barrels (e.g., two, four) according to the methodof the present invention. Carburetors with two or four barrels each havetwo primary passages or jets. Each primary jet has its own idle mixturescrew. One additional factor that must be considered in adjusting thesemultiple barrel carburetors is that it is important that each of theidle mixture screws be balanced, so that the same amount of fuel enterseach mixing chamber. To insure proper balance, each idle mixture screwis set at the same relative position. One method of insuring that theidle mixture screws are in the same relative position is to turn eachscrew so that it becomes lightly seated against the body of thecarburetor. Each screw is then turned in the opposite direction a setnumber of turns. This procedure may be followed for each idle mixturescrew in turn in order to avoid causing the engine to stop from completelack of fuel (this will occur if all the idle mixture screws are lightlyseated at the same time). Not only does this procedure balance the idlemixture screws, but also it may help to free the carburetor jets ofdeposits and other dirt. When making adjustments to the screw positions,each screw should be moved the same amount. The same criterion for aminimum emissions adjustment applies to the multiple barrel carburetorsas to the single barrel carburetors.

The following example illustrates the method of the present invention.However, this illustration is not to be interpreted as a specificlimitation on this invention.

EXAMPLE 1 A 1968 Oldsmobile automobile powered by 455 cubic inchdisplacement, eight-cylinder, four-cycle, internal combustion engineequipped with a two-barrel carburetor was chosen for formance, thecarburetor was properly adjusted. Exhaust gas hydrocarbon concentrationswere obtained by sampling the tailpipe effluent and analyzing forhydrocarbons by means of a portable electronic nondispersive infraredanalyzer.

After each idle mixture screw adjustment and before the collection ofthe corresponding sample of exhaust gas for hydrocarbon analysis, theengine was run at an elevated speed (approximately 2,000 rpm.) for about30 seconds to clear the engine exhaust system and to insure thecollection of a representative sample of exhaust gas. The initial idlehydrocarbon analysis indicated that the engine exhaust gas contained 430p.p.m. hydrocarbon. Both idle mixture screws were adjusted equally,about one-half turn to reduce the amount of fuel to the engine. Ahydrocarbon analysis indicated that the exhaust gases at this pointcontained 380 p.p.m. hydrocarbon. The fuel proportion of the fuel-airmixture was again reduced by turning each of the idle mixture screwsone-half turn and the hydrocarbon concentration of the exhaust gases wasfound to be 340 p.p.m. The fuel proportion was again reduced (byadjusting each screw one-half turn) and the hydrocarbon concentrationwas 300 p.p.m. Further fuel proportion reductions (by adjusting eachscrew onefourth turn) resulted in exhaust gas hydrocarbon concentrationsof 290 p.p.m., 280 p.p.m., 260 p.p.m., 270 p.p.m., 290 p.p.m., 300p.p.m., and 360 p.p.m., at which point the engine was noticeably roughin operation. The idle mixture screws were adjusted back to the pointwhere the resulting fuel proportion gave an exhaust gas hydrocarbonconcentration of 260 p.p.m.

With the exhaust gas hydrocarbon concentration at 260 p.p.m., the engineoperated smoothly and efficiently. The reduction in hydrocarbonconcentration from the initial analysis was p.p.m. or more than 39percent. The initial exhaust gas carbon monoxide concentration was about7.7 percent by volume, while the engine exhaust gas with the carburetoradjusted as above contained 1.4 percent by volume carbon monoxide. Thisrepresented a reduction of about 68 percent in exhaust gas carbonmonoxide concentration.

As the above emission reduction data indicate, the present invention canresult in a significant improvement in the amounts of hydrocarbon andcarbon monoxide emitted from internal combustion engines. This reductionin the amount of these han'nful pollutants is achieved withoutsacrificing the operating efficiency of the internal combustion engine.

While this invention has been described with respect to various specificexamples and embodiments, it is to be understood that the invention isnot limited thereto and that it can be variously practiced within thescope of the following claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A method for minimizing the amount of hydrocarbons emitted in theexhaust gas from an internal combustion engine which utilizes acarburetor to provide an air-fuel mixture to said engine, saidcarburetor being equipped with at least one idle mixture screw theposition of which may be adjusted to vary the air-fuel ratio of saidmixture, which method com- P118881 1. seating said idle mixture screwagainst the body of said carburetor;

2. adjusting the position of said idle mixture screw so as to provide anoverly-rich air-fuel mixture to said engine;

'3. adjusting the engine idle speed to an essentially constant speed onthe idle carburetion circuit in the range from about 400 rpm. to about1,100 r.p.m. analyzing for the hydrocarbon concentration of the exhaustgases from said engine collected while said engine is operated atessentially constant conditions on the idle carburetion circuit atnormal operating temperatures;

5. adjusting the position of said idle mixture screw by a smallincrement to reduce the proportion of fuel in said air-fuel mixture;

6. repeating step (4) with the air-fuel ratio being as adjusted in step(5);

7. repeating steps (5) and (6) at least until an analysis shows anincrease in hydrocarbon concentration over that obtained in theimmediately preceding analysis; and

8. setting the position of said idle mixture screw so that thecorresponding air-fuel ratio gives an exhaust gas hydrocarbonconcentration within the range from zero to about 50 p.p.m. above theminimum hydrocarbon concentration observed in steps (4), (6) and (7).

2. The method of claim ll wherein step (8) comprises maintaining theposition of the idle mixture screw as last adjusted in step (7).

3. The method of claim 1 wherein step (8) comprises setting the positionof the idle mixture screws so that the corresponding air-fuel ratiocontains a maximum proportion of fuel and provides an exhaust gashydrocarbon concentration within a range from zero to about 50 p.p.m.above the minimum hydrocarbon concentration observed in steps (4), (6)and (7 4. The method of claim 1 wherein step (8) comprises setting theposition of the idle mixture screws so that the corresponding air-fuelratio provides the minimum exhaust gas hydrocarbon concentrationobserved in steps (4), (6) and (7).

5. The method of claim 1 wherein said small increment is less than aboutone-half turn.

6. The method of claim 4 wherein step (8) comprises setting the positionof the idle mixture screws so that the corresponding air-fuel ratiocontains a minimum proportion of fuel and provides the minimum exhaustgas hydrocarbon concentration observed in steps (4), (6) and (7).

7. The method of claim 3 wherein said exhaust gases are

1. A method for minimizing the amount of hydrocarbons emitted in theexhaust gas from an internal combustion engine which utilizes acarburetor to provide an air-fuel mixture to said engine, saidcarburetor being equipped with at least one idle mixture screw theposition of which may be adjusted to vary the air-fuel ratio of saidmixture, which method comprises:
 1. seating said idle mixture screwagainst the body of said carburetor;
 2. adjusting the position of saididle mixture screw so as to provide an overly-rich air-fuel mixture tosaid engine;
 3. adjusting the engine idle speed to an essentiallyconstant speed on the idle carburetion circuit in the range from about400 r.p.m. to about 1,100 r.p.m.;
 4. analyzing for the hydrocarbonconcentration of the exhaust gases from said engine collected while saidengine is operated at essentially constant conditions on the idlecarburetion circuit at normal operating temperatures;
 5. adjusting theposition of said idle mixture screw by a small increment to reduce theproportion of fuel in said air-fuel mixture;
 6. repeating step (4) withthe air-fuel ratio being as adjusted in step (5);
 7. repeating steps (5)and (6) at least until an analysis shows an increase in hydrocarbonconcentration over that obtained in the immediately preceding analysis;and
 8. setting the position of said idle mixture screw so that thecorresponding air-fuel ratio gives an exhaust gas hydrocarbonconcentration within the range from zero to about 50 p.p.m. above theminimum hydrocarbon concentration observed in steps (4), (6) and (7). 2.adjusting the position of said idle mixture screw so as to provide anoverly-rich air-fuel mixture to said engine;
 2. The method of claim 1wherein step (8) comprises maintaining the position of the idle mixturescrew as last adjusted in step (7).
 3. The method of claim 1 whereinstep (8) comprises setting the position of the idle mixture screws sothat the corresponding air-fuel ratio contains a maximum proportion offuel and provides an exhaust gas hydrocarbon concentration within arange from zero to about 50 p.p.m. above the minimum hydrocarbonconcentration observed in steps (4), (6) and (7).
 3. adjusting theengine idle speed to an essentially constant speed on the idlecarburetion circuit in the range from about 400 r.p.m. to about 1,100r.p.m.;
 4. analyzing for the hydrocarbon concentration of the exhaustgases from said engine collected while said engine is operated atessentially constant conditions on the idle carburetion circuit atnormal operating temperatures;
 4. The method of claim 1 wherein step (8)comprises setting the position of the idle mixture screws so that thecorresponding air-fuel ratio provides the minimum exhaust gashydrocarbon concentration observed in steps (4), (6) and (7).
 5. Themethod of claim 1 wherein said small increment is less than aboutone-half turn.
 5. adjusting the position of said idle mixture screw by asmall increment to reduce the proportion of fuel in said air-fuelmixture;
 6. repeating step (4) with the air-fuel ratio being as adjustedin step (5);
 6. The method of claim 4 wherein step (8) comprises settingthe position of the idle mixture screws so that the correspondingair-fuel ratio contains a minimum proportion of fuel and provides theminimum exhaust gas hydrocarbon concentration observed in steps (4), (6)and (7).
 7. The method of claim 3 wherein said exhaust gases areanalyzed for hydrocarbon concentration by means of infraredspectrometry.
 7. repeating steps (5) and (6) at least until an analysisshows an increase in hydrocarbon concentration over that obtained in theimmediately preceding analysis; and
 8. setting the position of said idlemixture screw so that the corresponding air-fuel ratio gives an exhaustgas hydrocarbon concentration within the range from zero to about 50p.p.m. above the minimum hydrocarbon concentration observed in steps(4), (6) and (7).
 8. The method of claim 6 wherein said exhaust gasesare analyzed for hydrocarbon concentration by means of infraredspectrometry.
 9. The method of claim 7 wherein said small increment isless than about one-eighth turn.
 10. The method of claim 7 wherein saidessentially constant idle speed is a speed within the range of speedsrecommended by the manufacturer of said engine.